U.S. Pat. Ser. No. 4,188,661, "Direct Drive Ballast With Starting Circuit" by Bruce L. Bower and Raymond H. Kohler, dated Feb. 12, 1980, assigned to the assignee of the present invention, and hereby incorporated by reference, describes an electronic ballast circuit for driving a pair of fluorescent lamps. Central to the operation of that circuit is a high frequency (20 to 30 KHz) inverter comprising two transistors connected in series and operating in a push-pull mode. The inverter drives, via an output transformer, the cathode filaments of the lamps. The output transformer comprises a series-resonant primary winding coupled to the inverter output. The secondary of the output transformer includes one lamp voltage winding and three filament windings. Two filament windings separately supply current to one filament of each of the lamps. The third filament winding supplies current to the remaining two, parallel-connected, filaments. Also, included on the secondary of the output transformer is a series connected discrete ballasting inductor in series with a pair of bias windings oppositely poled and connected in series between the first and second filament windings. These windings are arranged so as to establish a voltage differential across the respective lamps sufficient to effect firing of the lamps.
The ballast circuit further includes an interstage transformer having three primary-wound feedback windings each coupled in a loop that includes at least one lamp filament and a filament winding. The secondary of the interstage transformer includes a pair of oppositely-poled drive windings coupled to the push-pull inputs of the inverter. Because the primary windings are coupled in a loop that includes the lamp filaments, they induce a voltage in a secondary proportional to the sum of filament currents. Proper phasing of the secondary windings provides the positive feedback necessary to sustain inverter operation. (A modified feedback arrangement disclosing a single primary winding connected in a loop with the two parallel-connected filaments is disclosed in U.S. Pat. Ser. No. 4,127,893, "Tuned Oscillator Ballast Circuit With Transient Compensating Means" by Charles A. Goepel and assigned to the assignee of the present invention. See FIG. 2 of that patent.)
U.S. Pat. Ser. No. 4,188,661 also discloses circuitry for enhancing the oscillator startup operation. Upon initial energization of the ballast circuit, a capacitor connector in parallel with one of the secondaries of the interstage transformer is charged through a source of slowly developed DC voltage. When the charge across the capacitor reaches a given magnitude, a series connected diac is switched on therby discharging the capacitor through a relatively low impedance and causing a transient across one of the drive windings of the interstage transformer. This perturbation supplies base drive to at least one of the inverter transistors and assures oscillator startup. A voltage derived from the current in the primary of the output transformer is applied to the diac in a manner that renders the diac nonconducting during steady state operation of the ballast circuit.
A related ballast circuit is disclosed in U.S. Pat. Ser. No. 218,311, cited above, and includes inter alia, an improved drive scheme for the transistorized inverter, a delayed starting circuit, a reconfigured output scheme and, in particular, a dimming circuit amenable to control from a remote location. (The dimming circuit disclosed in U.S. Pat. Ser. No. 218,311 may be deemed an alternative to, albeit in some respects an improvement upon, the dimming circuit disclosed in U.S. Pat. application Ser. No. 55,667, "Electronic Ballast Dimming Circuitry", filed July 9, 1979, now abandoned, by Gerald T. Smith and assigned to the assignee of this invention.) Dimming is effected by varying an inductance, and hence the total impedance, in a feedback loop that includes the primary of the output transformer and the transistor inverter. The variable inductance assumes the form of a saturable reactor, the effective inductance of which is varied according to the amplitude of a signal (DC current or voltage) applied to an associated control winding. As the effective inductance, i.e., impedance, of the saturable reactor is decreased, the amount of feedback applied to the inverter is increased and the power supplied to the lamps increased accordingly.
While it cannot be gainsaid that the circuitry disclosed therein represents a substantial advance in the state of the art of electronic ballast design, especially in that it provides remote dimming capability via a technique compatible with standard integrated circuit or computer-type control modules of modest power sourcing capacity, it will become clear that the subject invention represents yet another distinct advance in that art.